Layered reservoir

[strene]

This pull request introduces a new utility for constructing layered reservoir domains and refactors several case setup functions to use this utility, improving consistency and flexibility in handling layer properties. It also adds comprehensive tests for the new utility and tightens input validation for meshing. Minor improvements and citation updates are also included.

Layered domain utility and refactoring:

• Added layered_reservoir_domain function in src/cases/utils/layered_domain.jl to centralize mesh creation and layer property mapping for reservoir domains, supporting flexible property input and error handling.
• Refactored case setup functions (ags, btes, geothermal_doublet) in src/cases/ags.jl, src/cases/btes.jl, and src/cases/doublet.jl to use layered_reservoir_domain, simplifying property handling and mesh generation. [1] [2] [3]
• Updated file inclusion in src/cases/cases.jl to load the new utility module.

Testing and validation improvements:

• Added test/layered_domain.jl with extensive tests for layered_reservoir_domain, covering correct property mapping, return types, and error handling for invalid inputs.
• Tightened input validation in extruded_mesh (src/meshing/extruded.jl), enforcing that depths are sorted and that layer thicknesses and mesh parameters are consistent. [1] [2]

Minor improvements and fixes:

• Updated schedule tests to use the current year and pass start_year explicitly for reproducibility in test/cases.jl. [1] [2]
• Added a new conference citation for Fimbul.jl in CITATION.cff.

Case-specific adjustments:

• Adjusted well index assignment in geothermal_doublet

Miscellaneous:

• Minor formatting improvements in example scripts. [1] [2]
• Updated README citation formatting.
• Included new test file in test/runtests.jl.